Abstract
In the present investigation, the toughness of A1-8Si casting alloy has been evaluated by an instrumented Charpy impact test machine, using U-notched specimens. In attempting to identify the role of the microstructure in relation to toughness, a number of microstructural parameters, including dendrite spacing (λd), mean free path (MFP) across the dendrites (i.e., edge-to-edge dendrites spacing), in addition to Si particle size, morphology and spacing, are considered. It has been found that the microstructural change has affected the toughness of the alloy under investigation in two ways, namely an increase in the initiation energy required for fracture, and an increase in the propagation energy of fracture. The presence of fine Si particles of low aspect ratio and with a beneficial degree of sphericity has affected the initiation energy as a result of an improvement in the resistance to void initiation. In the case of propagation energy of fracture, the void size and its effect of local strain concentration has contributed to the superior values. In addition, the increase in the ratio (λ/DE)Si has also been found to play an influential part in improving toughness. An increase in the ratio (λ/DE)Si is expected to provide better resistance to the rate of void growth, with a consequent increase in toughness. The toughness values were also found to correlate well with a parameter φ (= MFP/λd) representing the structural integrity. Toughness appears to increase linearly with the decrease in φ. A fractographic study indicated that both the mode and pattern of fracture are dependent on the microstructural features of the Al-8Si alloy. The former showed a tendency towards transgranular fracture through the Al phase, whilst the latter changed from being brittle to being ductile. The change in the mode of fracture can be fairly attributed to the decrease in φ, or indeed to the decrease in the MFP. The change in the pattern of fracture can, with confidence, be attributed to the fineness of the Si particles which exhibit regular morphology and a larger (λ/DE)Si.
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